A variety of battery-driven electronic apparatuses, including a mobile phone, a digital camera, a tablet terminal, a portable music player, a portable game machine, a notebook computer, etc., contains a rechargeable battery (secondary battery) which supplies power required to operate a CPU (Central Processing Unit) for system control and signal processing, a liquid crystal panel, a radio communication module and other electronic circuits such as analog and digital circuits.
FIG. 1 is a block diagram of a battery-driven electronic apparatus. An electronic apparatus 900 includes a battery 902 and a charging circuit 904 which charges the battery 902. The charging circuit 904 charges the battery 902 with a power supply voltage VADP received from an external power adaptor and a USB (Universal Serial Bus).
A load 908 is connected to the battery 902. A current IBAT flowing into the battery 902 corresponds to a difference between a charging current ICHG from the charging circuit 904 and a load current (discharging current) ILOAD flowing into the load 908.
It is known that the battery 902 is degraded every time it is repeatedly charged/discharged. Specifically, the degradation of the battery 902 appears as a reduction of its capacity. The electronic apparatus 900 has a degradation estimation function to estimate the degradation of the battery 902. In some cases, a degradation estimation circuit 920 to estimate the degradation of the battery is integrated with a residual capacity detection circuit 906 to detect the residual capacity (state of charging; SOC) of the battery.
The residual capacity detection circuit 906 is called a fuel gauge IC (Integrated Circuit). A voltage method and a coulomb count method (charge integration method) are mainstream methods of detecting the battery residual capacity by means of the residual capacity circuit 906. In some cases, the residual capacity detection circuit 906 is incorporated in the charging circuit 904.
The voltage method is to measure an open circuit voltage (OCV) of the battery in an open state (no-load state) and estimate the residual capacity of the battery from the correspondence relationship between OCV and SOC. Since the OCV cannot be measured when the battery is not in a no-load relaxed state, the OCV cannot be correctly measured during the charging/discharging of the battery.
The coulomb count method is to integrate a charging current flowing into the battery and a discharging current flowing out of the battery (hereinafter these currents are collectively referred to as a charging/discharging current) and estimate the residual capacity of the battery by calculating the quantity of charges of charging and discharging of the battery, The coulomb count method can estimate the battery residual capacity even in a battery use period for which an open voltage cannot be obtained, unlike the voltage method.
The residual capacity detection circuit 906 shown in FIG. 1 estimates the residual capacity of the battery 902 according to the coulomb count method. The residual capacity detection circuit 906 includes a coulomb counter circuit 910 and an SOC calculation part 912. The coulomb counter circuit 910 detects and integrates the current IBAT of the battery 902. A coulomb count value (hereinafter referred to as an accumulated coulomb count (ACC)) determined by the coulomb counter circuit 910 is expressed by the following equation.ACC=∫IBATdt 
Speaking strictly, the battery current IBAT is discretely sampled on a time basis and is calculated according to the following equation.ACC=Σ(Δt×IBAT)
Where, Δt represents a sampling period.
This integration is performed with a current IBAT flowing out of the battery 902 as positive and a current IBAT flowing into the battery 902 as negative.
The SOC calculation part 912 calculates SOC of the battery 902 based on the calculated ACC. The SOC is calculated according to the following equation.SOC[%]=(CCFULL−ACC)/CCFULL×100
Where, CCFULL represents the quantity of charges (coulomb count capacity) accumulated in the battery 902 in a full-charged state and corresponds to the capacity of the battery 902.
A charging/discharging cycle detection part 922 of the degradation estimation circuit 920 detects use corresponding to one charging/discharging cycle based on data determined by the coulomb counter circuit 910. A degradation calculation part 924 increments a charging/discharging cycle number CYCCD every time the charging/discharging cycle detection part 922 detects one charging/discharging cycle. The charging/discharging cycle number CYCCD indicates a degree of degradation of the battery. The charging/discharging cycle number CYCCD can be used as an alarm to indicate the battery life.
In addition, the charging/discharging cycle detection part 922 corrects the coulomb count capacity (battery capacity) CCFULL used in the SOC calculation part 912 based on the charging/discharging cycle number CYCCD.
The present inventor has made studies on the degradation estimation circuit 920 of FIG. 1 and came to recognize the following problems. FIG. 2 is a graphical view showing a relationship between the charging/discharging cycle number CYCCD and the degree of degradation of the battery. The degradation degree represents a degraded battery capacity CCFULL with the rated capacity as 100%. As shown in FIG. 2, the relationship between the charging/discharging cycle number CYCCD and the degradation degree is varied depending on conditions such as battery temperature and the like. For example, the battery is degraded faster at a temperature of 45 degrees C. than at a temperature of 25 degrees C. On the other hand, the degradation calculation part 924 defines the relationship between the charging/discharging cycle number CYCCD and the degradation degree (degradation characteristics) under predetermined conditions (such as a predetermined temperature). Therefore, when the predetermined conditions differ from the actual battery use conditions, an error of degradation estimation increases.